1. Field of the Invention
This invention relates to a device for manufacturing single crystals by the CZ (Czochralski) method.
2. Description of Prior Art
Conventionally, single crystal silicon is produced by the CZ method in a single-crystal manufacturing apparatus shown in FIG. 6. The lower portion of the single-crystal manufacturing apparatus is a chamber 6, and a crucible 7 is disposed at the central portion of the chamber 6 in a manner capable of rotating and ascending/descending. The crucible 7 consists of a quartz crucible 7b and a graphite crucible 7a in which the quartz crucible 7b is accommodated. Polycrystalline silicon is deposited into the quartz crucible 7b so as to be heated to melt into a melted liquid 9 by way of a cylindrical heater 8 disposed around the crucible 7. Subsequently, a seed crystal installed within a seed holder 10 is dipped into the melted liquid 9. Afterward, the seed holder 10 and the crucible 7 are respectively driven to rotate in the same or reverse directions. At the same time, the seed holder 10 is lifted to grow a single-crystal silicon 5 (hereinafter referred as the single crystal) with predetermined diameter and length. Furthermore, a keep-warm cylinder 11 is disposed around the cylindrical heater 8.
To enhance the productivity of single-crystal manufacturing, it is necessary to rapidly cool down single crystals and to increase the lifting speed of single crystals. To reach this end, as shown in FIG. 6, a heat-shield plate 12 is disposed around the lifting zone of the single crystal 5. The heat-shield plate 12 consists of a ring-shaped rim 12a, an adiabatic duct 12b in the shape of a reversed truncated cone whose diameter decreases from its upper end toward its bottom. The heat-shield plate 12 is used for shielding the single crystal 5 from heat radiating from the crucible 7 or the heater 8. Especially, the heat-shield plate 12 is used for amplifying the temperature gradients in the longitudinal and the radial directions in the region near the solid-liquid boundary. Furthermore, the heat-shield plate 12 plays the role of guiding the inert gas coming from above the chamber 6 into the surroundings of the single crystal 5 and expelling gases hindering single-crystallization, such as SiO, SiO.sub.2, Si, or metal vapors coming from the graphite crucible 7a. This can improve the feasibility of dislocation-free crystals.
However, in the process of lifting single crystals by utilizing conventional heat-shield plates, it is not sufficient to shield the radiation heat coming from melt surface, the heater, or crucible to the single crystal. Thus, the lifting speed is restrained. To solve the problem, a cooling body (hereinafter referred as an after-cooler) consisting of cooling pipes or cooling jackets surrounding the single crystal being lifted disposed between the single crystal and the heat-shield plate has been suggested. However, the after-cooler should be made of material with high coefficient of thermal conductivity and not cause any contamination to the melted liquid or single crystals.
Conventional after-coolers installed in single-crystal manufacturing devices, for example, the cooling cylinder installed in the single-crystal manufacturing device disclosed in JP-A 4-317491 (JP-A: Japanese unexamined Patent Publication), comprise an outer cylinder facing the heater made of material having a low emissivity value, such as mirror-polished SUS or Molybdenum, and an inner cylinder facing the single crystal being lifted and fabricated from parts having a high emissivity value, such as copper pipes warped into a spiral shape. Furthermore, as described in the single-crystal manufacturing device disclosed in JP-A 8-239291, an after-cooler comprises a duct system for circulating liquid coolant, and a cooling body disposed underneath the duct system. The cooling body is made of silver or at least coated with silver. In alternative embodiments, the inner surface of the after-cooler facing the single crystal can be coated with black color so as to absorb the incident radiation heat, and the outer surface of the after-cooler facing the heat-shield plate may be, for example, mirror-polished or overlaid with gold so as to reflect the incident radiation heat.
However, the above conventional after-cooler has the following drawbacks:
(1) In the cooling cylinder installed in the single crystal manufacturing device disclosed in JP-A 4-317491, copper pipes are used as the inner cylinder. However, it is known that copper forms nuclei of crystal defects. If copper is used as the material of after-coolers, it will become a contamination source within the furnace. This will reduce the device yield. PA1 (2) In the single-crystal manufacturing device disclosed in JP-A 8-239291, the outer surface of the cooling body facing the heat-shield plate is mirror-polished or overlaid with gold so as to reduce its emissivity value. The heat-absorption capacity of the above cooling body is lower than that of the cooling body made of high-emissivity-value material. This keeps the surroundings of the heat-shield plate and the upper end portion of the heater at a high temperature. As a result, the temperature gradient within the hot zone is reduced and the temperature gradient in the single crystal axis decreases. Accordingly, the crystal lifting speed declines.